1. Field of the Invention
This invention relates to a self-processing type film unit for an instant camera in which developing solution contained in the film unit is distributed over an image forming area thereof after exposure to light to develop it on the spot.
2. Description of the Prior Art
As is well known, a self-processing type film unit for an instant camera generally includes a developing solution container disposed near the leading end thereof and means for trapping excess developing solution disposed near the trailing end thereof. The instant camera is arranged to feed the film unit after exposure to light in between a pair of spreading rollers which are rotated while nipping the film unit to discharge it outside the camera body. When the leading end portion of the film unit is passed between the spreading rollers, the developing solution container disposed therein is broken by the nipping force of the rollers and the solution is released. The released solution is spread over the image forming area which has been exposed, being pushed by the spreading rollers. In the conventional self-processing type film unit, there has been a problem that a part of excess developing solution which is a strong alkali and black in color leaks out of the film unit to contaminate the skin and/or clothes of the user, or the spreading rollers. When the spreading rollers are contaminated by the developing solution, the next several film units are also contaminated and the clearance between the rollers is changed to affect the spreading characteristics of the solution.
FIGS. 1 to 4 show a typical conventional self-processing type film unit. The film unit comprises a base sheet 2 bearing thereon a photosensitive layer 1, a masking member 4 which has an opening 3 defining an image forming area and is superposed on the base sheet 2, and a transparent cover sheet 5 superposed on the masking member 4 to cover the opening 3. The masking member 4 is longer than the base sheet 2 to extend beyond the ends of the base sheet 2. On the end portion 6 of the masking member 4 extending beyond the leading end (the film unit is fed in between the spreading rollers of the instant camera in the direction of the arrow A) of the base sheet 2 is disposed a developing solution container 7 and the end portion 6 is folded back over the cover sheet 5 along a folding line 10 with the solution container 7 being held between the upper run and the lower run of the masking member 4 as shown in FIG. 2. Then, the outer edge and the side edges of the end portion 6 are secured to the upper surface of the cover sheet 5. The cover sheet 5 has extensions 12a and 12b on the side edges 13a and 13b extending beyond the leading end 14 thereof, and the side edges of the end portion 6 of the masking member 5 are respectively secured to the extensions 12a and 12b near the folding line 10. When the cover sheet 5 is not provided with such extensions, the side edges of the end portion 6 may be secured to the side edges of the lower run of the masking member 4 near the folding line 10. The other end portion 8 is folded around a developing solution trapping member 9 along a folding line 11 onto the upper surface of the cover sheet 5 and is secured thereto at the outer edge and the side edges. The cover sheet 5 is secured to the masking member 4 only at the side edges 13a and 13b with the leading end 14 and the trailing end 15 being not secured to the masking member 4. Accordingly, the space in the bag-like portion formed by the end portion 6 of the masking member 4 at the leading end of the film unit communicates with the space in the bag-like portion formed by the end portion 8 at the trailing end of the film unit by way of the space between the cover sheet 5 and the base sheet 2, or more strictly by way of the spaces between end portions of the cover sheet 5 and the masking member 4 and between the intermediate portion of the cover sheet 5 and the intermediate portion of the base sheet 2. The thickness of the space between the cover sheet 5 and the base sheet 2 constitutes one of the factors which determine the spread state of the developing solution which is spread over the image forming area when the container 7 is broken by the spreading rollers. Therefore, the side edges 13a and 13b of the cover sheet 5 are attached to the masking member 4 sandwiching therebetween elongated spacers 16a and 16b in order to obtain optimum thickness of the space between the cover sheet 5 and the base sheet 2.
FIGS. 2 shows a film unit 17 in the assembled state. After the photosensitive layer 1 is exposed to light through the transparent cover sheet 5 and the opening 3 of the masking member 4, the film unit 17 is fed in between a pair of spreading rollers 18a and 18b of the instant camera rotating in the directions of the arrows B from its leading end portion containing therein the developing solution container 7. When the leading end portion of the film unit 17 is passed through the spreading rollers 18a and 18b, the developing solution container 7 is broken by the nipping force of the rollers 18a and 18b and the developing solution 7' contained therein is released. The released developing solution 7' is spread or distributed in the space between the cover sheet 5 and the base sheet 2 over the image forming area. Excess solution spread beyond the image forming area is trapped by the solution trapping member 9 disposed in the trailing end portion of the film unit 17.
Observations of the inventors have revealed that the excess developing solution leaks out of the film unit 17 through air escape holes 19 (FIG. 3) provided in the masking member 4 or through spaces 21 formed between the inner surface of the masking member 4 and the trailing end faces 5c and 16c of the cover sheet 5 and the spacer 16a (16b) at both sides. Since the cover sheet 5 and the spacers 16a and 16b have thickness, and the masking member 4 (generally formed of a plastic film) has rigidity, the space 21 is inherently formed between the trailing end faces 5c and 16c of the cover sheet 5 and the spacer 16a (16b) and the inner surface of the folded portion of the masking member 4 at each side of the unit (See FIG. 4). Generally, the width of the space 21 can be held smaller than 0.2 to 0.3 mm. By selecting the volume of the solution container 7 and the distribution of the solution in the container 7 taking into account the space 21, the leak of the solution through the space 21 can be prevented. In fact, the volume of the container 7 and the distribution of the solution therein have been strictly adjusted intending to prevent the leak of the solution. However, in order to prevent the leak of the solution, the volume of the container 7 and the solution distribution therein must be controlled with a high accuracy on the order of 0.01 cc. This, in addition to the fact that the width of the space 21 must be kept smaller than 0.2 to 0.3 mm, makes manufacture of the film units very difficult. Further, even if the width of the space 21 and the container 7 can be formed with the required high accuracy, still there is the possibility of the leak of the solution especially when the camera is used under high temperature and high humidity.
When the developing solution is spread over the image forming area in the pattern shown in FIG. 5A in which the solution is spread near the center of the unit 17 faster than near the edges of the unit, there is less possibility of the leak of the solution through the space 21. Recently, an attempt has been made to spread the solution in a relatively square pattern rather than the tongue-like pattern shown in FIG. 5A in order to evenly distribute the solution over the image forming area and to reduce the amount of excess solution. A pair of examples of the square solution spread pattern are shown in FIGS. 5B and 5C. When the solution is spread over the image forming area in a relatively square pattern, there is more possibility of the leak of the solution through the space 21. This is because a larger amount of solution is spread near the edges of the unit 17 and excess solution reaches the spaces 21 at the trailing end of the unit 17. Therefore, the solution spread pattern of FIG. 5C cannot be used so long as the space 21 exists even though the pattern may be preferred in order to obtain better reproduced image quality.